UNIVERSITY  OF   CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


THE    CALIBRATION    OF   THE 
LEAKAGE    METER 

FUMIGATION    STUDIES   No.  8 

BY 
C.  W.  WOODWORTH 


BULLETIN  No.  264 

Berkeley,  CaL,  January,  1916 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY 

1916 


Benjamin  Ide  Wheeler,  President  of  the  University. 
EXPERIMENT    STATION    STAFF 

HEADS   OF   DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 

Edward  J.  Wickson,  Horticulture   (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;  Plant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

William  A.  Setchell,  Botany. 

Myek  E.  Jaffa,  Nutrition. 

Eobert  H.  Loughridge,  Soil  Chemistry  and  Physics   (Emeritus). 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T   Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence   M.   Haring,  Veterinary   Science   and   Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

A.  V.  Stubenrauch,  Pomology. 

Walter  Mulford,  Forestry. 

W.   P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

D.  T.  Mason,  Forestry. 

J.  B.  Davidson,  Agricultural  Engineering. 

Elwood   Mead,   Rural  Institutions. 

H.  S.  Reed,  Plant  Physiology. 

William  G.  Hummel,  Agricultural  Education. 

Leon  M.  Davis,  Dairy  Industry. 

John  E.  Dougherty,  Poultry  Husbandry. 

S.  S.  Rogers,  Olericulture. 

Frank  Adams,  Irrigation  Practice. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

division  of  entomology 

C.  W.  Woodworth  E.  O.  Essig 

W.  B.  Herms  S.  B.  Freeborx 

E.  C.  Van  Dyke  E.  P.  Van  Duzee 

Geo.  A.  Coleman  M.  R.  Miller 

Geo.  P.  Gray  E.  R.  de  Ong 


THE  CALIBRATION  OF  THE  LEAKAGE  METER 

By  C.  W.  WOODWORTH 


Some  of  the  leakage  meters  put  on  the  market  prove  to  have  been 
incorrectly  calibrated  and  indicate  excessive  leakages.  This  has  not 
been  serious  as  long  at  the  instruments  have  been  used  only  for  the 
comparison  of  samples  of  tent  materials,  but  they  should  be  corrected 
to  suit  them,  for  use  in  connection  with  dosage  tables.  This  is  easily 
done  by  using  a  reamer  on  the  small  opening  from  the  brass  tube  to 
to  the  small  chamber  behind  the  cloth  under  the  clamp.  It  requires 
very  skillful  work,  however,  and  it  is  wise  to  return  to  the  manufacturer 
for  adjustment  any  meters  that  do  not  register  correctly. 

The  present  publication  is  intended  to  enable  users  of  these  instru- 
ments to  determine  approximately  the  correctness  of  this  adjustment. 
The  theory  of  the  meter  is  that  if  two  test  plates  were  used  to  corre- 
spond with  the  two  thicknesses  of  cloth  to  be  tested  then  the  diameters 
of  the  test  plates  would  bear  such  a  relation  to  the  diameter  of  the 
sharp  edge  of  the  clamp  that  the  areas  of  the  smaller  circle  in  the 
test  plate  should  be  between  .20  and  .25  per  cent  of  the  area  of  the 
larger  circle  of  the  clamp  and  precisely  the  amount  shown  by  the  test 
line  on  the  meter.  Instead,  however,  of  using  two  test  plates  a  single 
plate  with  a  smaller  hole  of  exactly  the  right  size  to  make  the  same 
reading  on  the  meter  is  furnished  with  the  instrument.  The  smaller 
hole  is  just  84  per  cent  of  the  diameter  that  would  give  the  right 
calculation  of  percentages  of  leakage  through  the  doubled  cloth, 
therefore  for  making  calculations  the  diameters  of  the  test  plates  must 
be  increased. 

The  testing  of  the  meter  consists  in  measuring  the  size  of  the  ring 
of  the  clamp  and  of  the  hole  in  a  test  plate  and  finding  whether  or 
not  the  actual  test  corresponds  with  the  percentages  calculated  from 
the  measurements  of  these  sizes.  Accurate  testing  requires  very  careful 
work  but  it  is  very  easy  to  make  a  test  sufficiently  accurate  to  determine 
whether  the  instrument  shows  gross  errors. 

MEASURING    THE    CLAMP 

The  best  way  to  measure  the  clamp  is  to  use  a  piece  of  cardboard 
with  which  to  take  an  impression  of  the  ring,  putting  in  several 
thicknesses,  if  necessary,  to  obtain  a  clear,  sharp  mark.    After  obtain- 

[231] 


232 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


ing  the  impression  measure  this  carefully  with  a  finely  divided  rule 
determining;  the  nearest  64th  of  an  inch.  The  size  will  be  found  to 
be  approximately  %  or  4%4ths  of  an  inch  and  the  table  below  gives 
readings  for  possible  variations  for  a  sixteenth  of  an  inch  in  either 
direction.  A  64th  of  an  inch  variation  modifies  the  reading  by  less 
than  .02  of  a  per  cent. 


MEASURING   THE   TEST  PLATE 

The  hole  in  the  test  plate  is  very  much  more  difficult  to  measure  ac- 
curately. The  standard  wire  gauge  known  as  the  ' '  Brown  and  Sharpe ' ' 
gauge  is  to  be  had  in  every  machine  shop  and  is  perhaps  the  most 
accurate  measure  for  this  purpose.  Lay  this  on  the  test  plate  and 
examine  it  under  a  pocket  lens.  The  hole  should  appear  as  shown  in 
figure  1.     Another  method  is  to  make  a  test  plate  by  the  use  of  an 


Fig.  1. — The  way  the  hole  in  the  test  plate  should  appear  under  the  wire  gauze 
as  seen  through  the  microscope. 


ordinary  sewing  needle.  This  method  will  probably  prove  more 
accurate  with  most  people  than  the  use  of  a  wire  gauge  because  of  the 
practical  difficulty  of  making  the  comparisons  under  the  microscope. 
The  best  test  plates  made  in  this  manner  are  with  soft  metal,  like 
copper,  lead  or  zinc.  A  very  thin  sheet  of  these  metals  can  be  very 
easily  perforated.  Fair  results  can  be  secured  by  using  a  piece  of 
good  quality  writing  paper  or  cardboard.  The  fiber  in  the  softer 
cardboard  swells  out  and  partially  closes  the  hole  and  so  gives  low 
readings.  If  it  is  found  that  a  needle  fits  tightly  in  a  hole  after 
withdrawing  it  a  few  minutes  it  is  not  suitable. 

A  very  good  method  is  found  to  consist  in  perforating  a  sheet  of 
celluloid  in  rather  hot  water,  then  placing  the  celluloid  in  cold  water 
and  leaving  the  needle  in  position  until  the  celluloid  is  cool.  The 
hole  under  these  circumstances  proved  almost  if  not  quite  as  good 
as  in  metal. 


THE   CALIBRATION   OF   THE   LEAKAGE    METER 


233 


In  all  cases  thrust  the  needle  through  to  the  middle  so  as  to  have 
the  full  width  of  the  shaft. 

The  use  of  the  needle  is  so  simple  and  convenient  that  it  will 
probably  be  the  most  satisfactory  method  in  practice  to  make  a  new 
test  plate  rather  than  to  attempt  to  measure  the  one  furnished  with 
the  instrument. 

MAKING    THE    TEST 

The  test  plate  is  clamped  in  the  instrument  and  the  reading  care- 
fully made  in  the  same  manner  as  in  testing  tent  material. 

Comparing  this  with  the  calculation  given  in  the  table  in  the  column 
under  the  diameter  of  the  hole  in  the  test  plate  and  the  line  of  the 
diameter  of  the  clamp  ring  enables  one  to  determine  the  per  cent  of 
error.  When  the  table  values  differ  from  the  determined  value  the 
error  is  approximately  6  per  cent  for  each  line.  The  difference  of 
adjacent  columns  is  approximately  four  times  as  great. 

One  degree  on  the  meter  between  .15  and  .20  corresponds  with 
about  six  lines,  between  .20  and  .25  four  lines  and  from  .25  to  .35 
about  three  lines.  If  the  observed  error  is  greater  than  this,  the  instru- 
ment should  be  sent  to  the  maker  for  adjustment,  and  perhaps  even 
for  a  lesser  error,  rather  that  to  try  to  adjust  by  reading  "strong"' 
or  "weak." 


TABLE   FOE   TESTING  LEAKAGE   METERS 

Wire  gauge  No 20  21 

Needle  No 5  6 

Diameter   032  .028 

Augmented    038  .033 


Diameter  of  clamp 

36-64ths  .564  in.  14.3  mm 35 

37  .580  14.7            33 

38  .595  15.1             31.5 

39  .610  15.5  39.0  29.8 

40  .625  15.9  37.0  28.5 

41  .642  16.3  35.0  26.9 

42  .658  16.7  33.3  25.6 

43  .672  17.1  32.0  24.5 

44  .688  17.5  32.5  23.5 


22 

23 

24 

25 

7 

8 

9 

10 

.025 

.022 

.020 

.018 

.030 

.026 

.024 

.021 

ct  meter 

readings  X  100 

28 

21.7 

17.9 

14.5 

26.4 

20.4 

16.9 

13.6 

25.0 

19.5 

16.0 

13.0 

23.8 

18.5 

15.3 

12.4 

22.7 

17.6 

14.6 

11.7 

21.5 

16.7 

13.8 

11.2 

20.5 

15.9 

13.1 

10.6 

19.6 

15.2 

12.6 

10.1 

18.7 

14.5 

12.0 

CALCULATION   OF    THE    TABLE 


While  it  is  not  necessary  to  make  any  calculations  to  test  a  meter 
it  will  be  well  to  explain  the  calculation  of  the  above  table. 


234  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  easiest  method  of  doing  this  is  by  the  use  of  a  slide  rule,  the 
settings  of  which  are  as  follows: 

C  21  _  C  needle  diameter  C  elamp  diameter       A  v 

D  25  ~  D  ~       ~^~  D  ~       ~^~  =  B  t 

x  =  augmented  needle  diameter  to  allow  for'  doubled  tents, 
y  =  correct  meter  readings  of  per  cent  of  leakage. 


SUMMARY 

Some  leakage  meters  are  so  inaccurate  that  they  are  unsuitable 
for  use  as  guides  to  dosage. 

The  accuracy  can  be  easily  tested  by  measuring  the  clamp  ring  and 
the  hole  of  the  test  plate. 

Needles  can  be  used  to  make  test  plates  making  their  measurement 
unnecessary.  The  amount  of  the  inaccuracy  is  determined  by  com- 
paring the  reading  with  a  table.  If  found  inaccurate  the  instrument 
should  be  returned  to  the  maker  for  adjustment. 


